1. Field of the Invention
One embodiment of the invention relates to a flat-panel display device. For example, the flat-panel display device is preferably applied to a liquid crystal display device.
2. Description of the Related Art
A conventional liquid crystal display device has the following configuration. Specifically, a signal processor converts a Y/U/V digital video signal to R, G and B color video signals. Thereafter, a digital-to-analog converter converts each of these R, G and B color video signals into an analog signal, and then, supplies the analog signal to a display unit. In this case, R, G and B analog signals output from the signal processor are sampled once by a sample-and-hold circuit built into a source driver of the display unit. When a signal for one horizontal line is sampled, the sample signals are supplied all together to a horizontal line pixel designated by a gate driver via a gate circuit.
Recently, the limited lifetime of energy resources and conservation of the natural environment have attracted the world's attention. In view of such circumstances, there is a need to promote energy saving and power saving in electronic apparatuses. In order to achieve power saving of a flat-panel display device, there has been proposed a method of reducing the number of pixels driven in one video frame, for example. However, this is a factor of reducing image quality depending on a selected mode of a driven pixel.
Conversely, in order to prevent a reduction of the image quality, various techniques have been conventionally proposed (e.g., see Jpn. Pat. Appln. KOAKI Publication No. 2003-259386).
In the display device, the digital-to-analog converter and the sample-and-hold circuit usually operates in synchronism with each other. However, there is the case where a processing for converting the number of pixels is carried out to reduce the number of pixels of the input signal. The foregoing processing for converting the number of pixels has the following purposes. One is a purpose for aligning the number of pixels of the input signal with the number of pixels of the display itself. Another is a purpose for achieving the foregoing power saving. In such a case, a signal output from the digital-to-analog converter is not always sampled by the sample-and-hold circuit at a suitable timing. For example, in the display unit formed using polysilicon, a sample phase shifts from a predetermined phase position resulting from non-uniformity of products.
In this case, each of corresponding RGB pixel signals are not always supplied accurately to each of RGB display pixels forming one color pixel of the display unit. If the corresponding RGB pixel signals are not accurately supplied to the RGB display pixels, quality reduction and change color occurs.
In addition, if a shift exists in the sample phase, a sample point position sampling and holding a pixel signal is variously varied with respect to a change point of the pixel signal output from the digital-to-analog converter (switching position of pixel signal). As described above, the sample point position of the pixel signal is variously varied with respect to a change point of the pixel signal, and thereby, an influence of carrier component appears. The influence of carrier component is a factor of generating a noise component; as a result, an image quality is reduced.
In addition, if a sampling frequency is high and an amount of write of the pixel signal to the display unit is much, a driver for driving the display unit has high load, and power consumption increases.